1. Field of the Invention
This invention relates to a method and device for the continuous automatic monitoring and control of the tension to which yarn is subjected during its winding in a textile machine, in particular in an automatic bobbin winding unit.
2. Description of the Related Art
It is known that in the textile industry each package of yarn spun in a ring spinning machine is fed in the next production stage to an automatic bobbin winding unit, where it is rewound to form a bobbin of substantially cylindrical or frusto-conical shape. The yarn is firstly extracted from the underlying package by pulling, to then pass through a tensioning device to subject the yarn to a constant tension necessary for correct winding onto the bobbin under formation.
In a bobbin winding unit with a grooved drive roller, the tendency to increase winding speed to advantageously achieve increased production results in a considerable increase in yarn tensioning, to the extent that the yarn can undergo breakage. Moreover in practice, as is well known to the expert of the art, when a yarn tension level has been set its average value does not remain constant. It is well known that during the to-and-fro travel of the yarn as it distributes itself along the surface of the bobbin under formation, small short-lasting oscillations in the yarn tension can occur, but generally these are innocuous because they settle about an average value, it being necessary only to control the yarn tension at a preset constant average value. The reasons why the average tension does not remain constant include the increase in the diameter of the wound yarn on the bobbin under formation, the unwinding of the last part of the yarn wound on the feed package, and the presence of dust and trash on the slide guides at the various support points along the path of the yarn being transferred from the underlying package to the overlying bobbin. In addition the average tension is influenced by the type and nature of the yarn being wound and the temperature of the surrounding atmosphere, so that the yarn has to be tensioned taking account of the aforesaid operating parameters in order to establish tension values such that the yarn is always in an elastically taut condition. Otherwise, each time the yarn breaks by separation due to excessive tensioning, the winding operation has to be interrupted to effect a yarn joining process, the winding efficiency of the bobbin winding unit hence undergoing a considerable decrease. At the same time a poor quality bobbin is formed because of the presence of frequent knots, which give rise to difficulties in further operations involving the bobbin. Yarn braking washers are known which operate with brake shoes to generate, by means of a structure of known construction, a preliminary set tension in the travelling yarn. These known devices have various drawbacks. The yarn-braking washers acting on the yarn exert on them a braking force which cannot be exactly defined, and a change in the yarn tension cannot be easily achieved because of the aforesaid reasons. Moreover the known devices require constant and periodic maintenance.